Higher operating temperatures for gas turbine engines are continuously sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components within the engine must correspondingly increase. For this reason, the use of TBCs on components, such as combustors, high pressure turbine (HPT) blades and vanes, has increased in commercial and military gas turbine engines. The thermal insulation of a TBC enables such components to survive higher operating temperatures, increases component durability, and improves engine reliability. A TBC is typically a ceramic material deposited on an environmentally-protective bond coat overlying a metallic substrate to form what is often termed a TBC system. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings, such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or other element), and oxidation-resistant diffusion coatings, such as diffusion aluminides that contain aluminum intermetallics.
Ceramic materials, particularly binary yttria-stabilized zirconia (YSZ) ceramics, are often used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by air plasma spraying (APS), flame spraying and physical vapor deposition (PVD) techniques. TBCs formed by these methods have a lower thermal conductivity than a dense ceramic of the same composition as a result of the presence of microstructural features or defects and pores in the TBC microstructure. TBCs employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and other similar melting and evaporation deposition processes.
In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC has and maintains a low thermal conductivity throughout the life of the component, including high temperature excursions. However, the thermal conductivities of TBC materials, such as YSZ, are known to increase over time when subjected to the operating environment of a gas turbine engine. As a result, TBCs for gas turbine engine components are often deposited to a greater thickness than would otherwise be desirable. Multiple layers are thus often added to some YSZ TBCs to correct deficiencies resulting in unwanted increased thickness of the coating system. Alternatively, internally cooled components, such as blades and nozzles, must be designed to have higher cooling flow. However, the above solutions may be undesirable for reasons relating to cost, weight, component life and engine efficiency.
As illustrated above, a thermal barrier coating, such as YSZ, while being known for providing certain environmental protection benefits to a coating system may not provide other important properties needed for an adequate coating. Often, prior thermal barrier coatings provide specific protection or benefits in one particular area, but fail to provide benefits in other important areas required by thermal barrier coating systems.
In view of the above, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBCs are employed to thermally insulate components intended for more demanding engine designs. A TBC having multiple beneficial effects, such as a low thermal conductivity, strong resistance to erosion and impact, sufficiently long life and phase stability would allow for higher component surface temperatures and reduced coating thickness for the same surface temperature. Reduced TBC thickness, especially in applications like combustors often employing relatively thick TBCs, would result in a significant cost reduction and weight benefit.